Archaean mantle xenoliths

Fragments of the Archaean mantle -xenoliths from the subcontinental lithosphere As geochronological determinations on xenoliths from the subcontinental lithosphere become more robust, a consistent observation is emerging - that the subcontinental lithosphere beneath Archaean continental crust is very ancient (see Section 3.1.3.2). Recent studies... 

One particular class of Archaean mantle xenoliths has received special attention. These are eclogites recovered from the subcontinental lithosphere. Richardson et al. (2001) showed that some Archaean eclogite xenoliths have the trace element and Os-isotopic characteristics of a basaltic protolith and an isotopic history which shows a significant time gap between basalt generation and eclogite crystallization. These properties are typical of subducted ocean floor and indicate that the subcontinental lithosphere beneath the Kaapvaal Craton contains fragments of 2.9 Ga subducted ocean floor. 

Hanghoj K., Kelemen P. B., Bernstein S., Blustztajn J., and Frei R. (2(X)1) Osmium isotopes in the Wiedemann Fjord mantle xenoliths a unique record of cratonic mantle formation by melt depletion in the Archaean. Geochem. Geophys. Geosys. 2 (20010109) 2000GC000085. 

The co-ordinated Kaapvaal Project geochron-ological studies of crustal and mantle xenoliths reveal that both crust and mantle have experienced a multi-stage history, and that a simple view of cratonization as a discrete event is not a viable model for craton formation (Schmitz et al. 1998 Schmitz Bowring 2000 Moser et al. 2001). The lower crust in particular retains a comprehensive record of the tectonothermal evolution of the lithosphere. The study of lower-crustal samples has shown that much of the deep craton experienced a dynamic and protracted history of tectonothermal activity that is temporally associated with events seen in the surface record, including late Archaean magmatism (Ventersdorp) and even Proterozoic deformation (Namaqua-Natal) (Schmitz et al. 1998). Thermal events are... 

One of the important discoveries about the subcontinental lithosphere to come from xeno-lith and diamond inclusion studies is that beneath many Archaean cratons the subcontinental mantle is extremely ancient. The results of thermobarometry on these mantle xenoliths have been used to construct paleo-geotherms for the subcontinental mantle (Fig. 3.4) which show that Archaean subcontinental lithosphere is significantly cooler than more recent lithospheric mantle. The early results of Richardson et al. (1984) on diamond... 

Pearson D. G., Carlson R. W., Shirey S. B., Boyd F. R., and Nixon P. H. (1995b) The stabilisation of Archaean lithospheric mantle a Re-Os isotope study of peridotite xenoliths from the Kaapvaal craton. Earth Planet. Sci. Lett. 134, 341-357. 

The growing consensus that ancient crust is always underlain by ancient lithospheric mantle has been challenged recently by Wu et al. (2003). These authors report an unusual absence of xenoliths with Archaean ages beneath the Archaean North China Craton. They propose that in some cases, therefore, ancient subcontinental mantle can be removed from beneath ancient continental crust by delamination - a process which has previously been postulated but never demonstrated. The subject of mantle delamination is discussed more fully in Chapter 5, Section 5.5.2. 

A number of studies have used xenoliths from Archaean subcontinental lithosphere to make inferences about the oxidation state of the early mantle. For example Woodland and Koch (2003) showed that the subcontinental lithosphere beneath the Kaapvaal Craton displays a systematic decrease in oxygen fugacity with depth (Fig. 5.11). However, it should be remembered that the highly depleted nature of the Archaean subcontinental lithosphere means that it is atypical of the mantle as a whole and may not therefore be useful as an indicator of mantle redox conditions (Chapter 3, Section 3.1.3.2). 

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